You can do it two ways. First is on the car. Put the car on 4 jack stands, remove wheels, release the emergency brake and put it on neutral. Then take 80 grit or 120 grit sandpaper and lightly sand the surface of both sides of your rotors. You are shooting for your rotors to look like this.
Obviously this was how the rotor looked when you received it new but you get the idea. The second is to remove the rotor from the car and I can only see you having to do this if you are unable to do the backside of the rotor.
After which you will want to follow the bedding process instructed by the manufacturer for the rotors. This process is to remove the old pad material so that the new pad material can transfer easier to the rotor.
This is also recommended if you feel like the rotor is warped as with most rotors the warped feeling is actually uneven pad material on the rotor. I had this happen when I completed a session at an HPDE and stopped for an extended period of time with my foot on the brake. IDIOT!!!